The present invention relates generally to an optical recording apparatus for recording an image by scanning a light beam in parallel and in order for exposure. More specifically, the invention relates to an image signal modulation device for an optical printer for high definition printing employing an electronic photographic process.
In recent years, laser printers having a capability of high speed and high quality recording have become popular despite a slightly higher cost due to a complicated construction in comparison with other printers.
However, when line images formed with oblique lines or curved lines, such as those in graphs or drawings, are recorded by means of a laser printer, the lines can become rugged in appearance and thus cannot be recorded with satisfactory sharpness. Therefore, for such images, XY plotters are still frequently used. Since the XY plotters employ normal pen points (approximately 100 to 500 .mu.m in diameter), they cannot make fine recording as that which can be done using a laser primers (60 to 100 .mu.m in dot diameter). However, XY plotters are able to record the oblique lines or the curved lines with uniform thickness and satisfactory sharpness.
The reason is that the pen point moves along the oblique line or the curved line to draw sharp contour when the XY plotter is used for recording the oblique line or the curved line.
On the other hand, Japanese Patent Document No. JP-A-61-38922 discloses a laser primer which modifies a laser beam in a scanning direction and a direction perpendicular to the scanning direction (i.e., the subsidiary scanning direction). An ultrasonic wave light deflector is inserted between a laser beam source and a polygon mirror so as to enable deflection in the subsidiary scanning direction. In addition, there are printers having an electro-optical deflector, a galvanomirror, a piezoelectric element, to which a reflector mirror or lens is attached, and so forth.
On the hand, the beam emitted from a laser oscillator and converted into a parallel beam by a lens is displaced in the subsidiary scanning direction by means of the ultrasonic deflector. Discussion with respect to the ultrasonic deflector has been given in Shibayama: "Elastic Surface Wave Technology", the Institute of Electronics and Communication Engineers. pp 132-141 (1983).
Furthermore, Japanese Patent Document No. JP-A-62-275214 discloses a variable focusing element which employs a semiconductor laser as a light source, an ultrasonic deflector for a spot position control in the subsidiary scanning direction and an EO (electro-optic) effect as an elements for spot diameter variation.
In addition, Japanese Patent Document No. JP-A-60-165866 discloses an apparatus, in which the laser beam is repeatedly deflected in the subsidiary scanning direction with respect to each pixel (or dot) as a unit of recording for forming an image as an image information.
The above-mentioned prior art references have a common problem in drawing all oblique and curved lines except for horizontal and vertical lines. This problem can be solved by increasing the scanning density of scanning lines. However, to achieve a satisfactorily high density, it becomes necessary to solve the following problems. Therefore, the satisfactory density cannot be easily achieved.
The problems to be solved are:
(1) to increase the rotation speed of the polygon mirror;
(2) to make the spot diameter of the laser beam smaller; and
(3) to use a fine particle toner for developing.
In addition, when a line image constituted by oblique lines and/or curved lines, such as a graph or a drawing is drawn by the laser printer, since the recording unit is the pixel, the recording of the lines with satisfactory sharpness cannot be achieved because the lines are inherently drawn in rugged configuration unless the size of the pixel is reduced to be not susceptible by the human eye. Additionally, in the above-mentioned prior art, since the position of the beam spot corresponding to the recording dot cannot be moved along the edge of the oblique line or the curved line, it is not possible to achieve the recorded image quality of the oblique line or the curved line as that can be achieved in recording an XY plotter. A similar problem arises in the case where an image other than a line image is drawn in addition to the line image. Therefore, progressively increasing needs have developed for drawing oblique and curved lines by a laser printer with equivalent quality to that achieved by the above-mentioned plotter.
In the above-mentioned prior art employing the plotter, although the above-mentioned advantage can be achieved, it can be defective due to a lack of resolution or a low recording speed in recording an image other than a line image. Therefore, such a plotter is not practically useful for recording an image other than a line image. For instance, unless the pen point is made sufficiently small, smaller characters can be defaced. Also, when a half-tone image is drawn as an image constituted with half-tone dots, dots must be drawn in a one-by-one basis, requiring a terribly long period of time.